When would you choose UDP over TCP?

When timeliness or low overhead matters more than guaranteed delivery, or when TCP's model doesn't fit. Real-time media (voice, video, telemetry) prefers UDP because a retransmitted late packet is useless, by the time it arrives the moment has passed, and TCP's head-of-line blocking would stall the stream waiting for a resend; it's better to drop the lost frame and keep going, optionally concealing the loss. DNS uses UDP because a query/response is a single small exchange where setting up a TCP connection would add round-trips for nothing. Multicast and broadcast require UDP, since TCP is strictly point-to-point. And constrained IoT devices favor UDP because TCP's per-connection buffers and state cost scarce RAM, and the handshake costs energy on a battery radio, which is why CoAP runs over UDP and adds its own lightweight, optional reliability. The general rule: pick UDP when you can tolerate (or handle yourself) loss/reordering and want minimal latency, overhead, and state; pick TCP when correctness and ordering are non-negotiable and you can afford the resources.